The locking of intramedullary nails is known in the art. One or more locking screws or locking bolts (in the following, only the expression, a locking screw, is used; however, it is intended to include also the expression, locking bolt) are inserted into the transverse boreholes of the intramedullary nail either with the help of an imaging method (X-ray control) or a relatively complicated targeting device. In both cases, a certain targeting inaccuracy cannot be avoided, that is, the tip of the screw cannot be aligned precisely coaxially with the central axis of the transverse borehole and, instead, deviates therefrom by a certain amount. The external diameter of the screw is underdimensioned relative to the diameter of the transverse borehole so that, in spite of this targeting error, the locking screw ends up in the transverse borehole and can be brought through this borehole. If the targeting inaccuracy remains within the scope of this underdimensioning, the locking screw can be passed through the transverse boreholes without any problems in spite of this targeting error. However, because of the underdimensioning, the locking screw now has a certain clearance relative to the transverse borehole.
This clearance defines the amount by which the main bone fragments, which are to be locked by means of the locking screws in the corresponding locking hole, can move relative to the nail and accordingly, because of the rigidity of the nail, also relative to the other main fragments of the bone fastened with the same nail. Admittedly, this clearance is unavoidable in order to guarantee the applicability of the locking for the surgeon. However, it is clinically undesirable for certain indications, for example, in the case of metaphysical fragments.
Even nails with a solid cross-section, which may have an internal thread in the locking hole, are not free of clearance. The internal thread merely prevents the nail from shifting axially on the locking screw.
From U.S. Pat. No. 6,296,645, a hollow, intramedullary metal nail is known, which has one or two plastic inserts, through which the locking screw can be introduced, in the casing openings of the transverse borehole, which are diametrically opposite to one another and are referred to as windows. It is a disadvantage of this known medullary nail that the window-like plastic inserts can be pressed in under the stresses occurring during clinical use, so that the desired function is lost. However, even if the manipulations are carried out very carefully, the two plastic inserts could be pressed out of their “window” when the locking screw is passed through, which also leads to a loss of function.